The present application is a U.S. non-provisional application based upon and claiming priority from Japanese Application No. HEI 10-363458, which is hereby incorporated by reference.
The present invention relates to a reaction apparatus with which it is possible to manufacture a polycarbonate that is not corroded by sulfur compounds contained in minute quantities in the polycarbonate manufacturing raw materials, that has a low content of foreign matter, and that has excellent coloring.
Because of their excellent impact resistance and other mechanical properties, as well as their heat resistance, transparency, and so on, polycarbonates are widely used in various machinery parts, optical disks, automotive parts, and other such applications. They are particularly promising for optical applications, such as memory-use optical disks, optical fibers, and lenses, and research into these materials is very active.
Known methods for manufacturing these polycarbonates include a method in which a bisphenol such as bisphenol A is directly reacted with phosgene (interfacial method), and a method in which a bisphenol such as bisphenol A and a carbonic diester such as diphenyl carbonate are subjected to melt polycondensation (transesterification).
Of these, the interfacial method, in which phosgene is used, is most commonly employed at the present time. Meanwhile, the melt polycondensation method has the advantage of allowing a polycarbonate to be manufactured less expensively than the interfacial method, and because it does not involve the use of a toxic substance such as phosgene, it is very promising as a polycarbonate manufacturing method.
When a polycarbonate is manufactured by this melt polycondensation method, bisphenol A (melting point: 156.degree. C.) and diphenyl carbonate (melting point: 80.degree. C.) are heated and melted, either separately or after being mixed, and a catalyst is added to the mixture of these two compounds, after which the system is heated to the reaction temperature and subjected to a polycondensation reaction.
Because the raw materials and the product polymer are exposed to a high temperature for an extended period with this melt polycondensation method, a problem that is encountered is that the obtained polycarbonate is susceptible to discoloration, and improvement is needed in this respect particularly with optical-use polycarbonates because they need to have little yellowness and excellent transparency.
There has been a proposal for a method for manufacturing a polycarbonate by conducting the transesterification in a reaction apparatus whose surfaces that come into contact with the raw materials are composed of nickel or the like so as to suppress the discoloration of the polycarbonate (see U.S. Pat. No. 4,383,092).
The inventors of the present invention have also worked on developing a polycarbonate material by transesterification using a reaction apparatus made of nickel, but noticed that part of the reaction apparatus becomes corroded when polycondensation of a polycarbonate is conducted continuously for an extended period. Another problem they encountered is that the metallic microparticles produced by this corrosion increase the quantity of foreign matter in the finished product and cause discoloration.
As a result of diligent research conducted in light of the above problems, the inventors discovered that the cause of the corrosion of the nickel contact surfaces in a polycarbonate reaction apparatus is a reaction between the nickel and the sulfur compounds such as 3-mercaptopropionic acid used as an auxiliary catalyst during the manufacture and contained in extremely small amounts in the raw material bisphenol A.
Upon further investigation, the inventors discovered that at 250.degree. C. or lower the nickel contact surfaces are less prone to corrosion because the reaction between the sulfur compounds and the nickel proceeds more slowly, but if the temperature is over 250.degree. C., the reaction between the sulfur compounds and the nickel proceeds much faster, as does the corrosion of the nickel contact surfaces, and equipment such as a heat exchanger is severely corroded on its interior.
As a result of further investigation conducted on the basis of the above knowledge, the inventors perfected the present invention upon discovering that if the reaction equipment surfaces that come into contact with the molten raw materials and the reaction product (a polycarbonate) at a temperature over 250.degree. C. are made from stainless steel having a nickel content of 5 to 15% and a chromium content of 10 to 20%, these surfaces will be unaffected by sulfur compound corrosion, and furthermore the discoloration of the finished product polycarbonate can be suppressed.